Deinococcus radiodurans is a non-sporeforming bacterium notable for its capacity to tolerate exposure to ionizing radiation (Battista and Rainey, Phylum BIV. “Deinococcus-Thermus” Family 1. Deinococcaceae Brooks and Murray 1981, 356,vp emend. Rainey, Nobre, Schumann, Stackebrandt and da Costa 1997, 513. In: Boone D R, Castenholz R W, editors, Bergey's Manual of Systematic Bacteriology, 2nd ed. New York: Springer, pp. 395-414, 2001). The D37 dose for D. radiodurans R1 is approximately 6500Gy, at least 200-fold higher than the D37 dose of E. coli cultures irradiated under the same conditions. The energy deposited by 6500Gy γ radiation should introduce thousands of DNA lesions including hundreds of double strand breaks (Smith, et al., Molecular biology of radiation resistant bacteria, In: Herbert R A, Sharp R J, editors, Molecular biology and biotechnology of extremophiles, New York: Chapman & Hall, pp. 258-280, 1992). The mechanisms responsible for this species' resilience are poorly described and recent analyses of DNA damage-induced changes in the proteome (Lipton, et al., Proc. Natl. Acad. Sci. USA 99(17):11049-11054, 2002) and transcriptome (Liu, et al., Proc. Natl. Acad. Sci. USA 100(7):4191-4196, 2003) of D. radiodurans cultures have done little to improve our understanding of D. radiodurans' radioresistance (Edwards and Battista, Trends Biotechnol. 21(9):381-382, 2003; Narumi, Trends Microbiol. 11(9):422-425, 2003).
For most species, the intracellular generation of strand breaks has lethal consequences; exposed free ends serving as substrates for intracellular exonucleases that degrade the genome. However, in D. radiodurans the presence of strand breaks does not result in a catastrophic loss of genetic information (Dean, et al., Nature 209(18):49-52, 1966; Lett, et al., Proc. R. Soc. Lond. B. Biol. Sci. 167(7):184-201, 1967; Vukovic-Nagy, et al., Int. J. Radiat. Biol. Relat. Stud. Phys. Chem. Med. 25(4):329-337, 1974). Instead this species appears to have the ability to control DNA degradation post-irradiation by synthesizing proteins that prevent extensive digestion of the genome, and it has been suggested that the DNA degradation observed in this species is an integral part of the process of DNA repair, generating single-stranded DNA that promotes homologous recombination and restitution of the damaged genome (Battista, et al, Phylum BIV. “Deinococcus-Thermus” Family 1. Deinococcaceae Brooks and Murray 1981, 356,vp emend. Rainey, Nobre, Schumann, Stackebrandt and da Costa 1997, 513. In: Boone D R, Castenholz R W, editors. Bergey's Manual of Systematic Bacteriology. 2nd ed. New York: Springer, pp. 395-414, 1999).
When D. radiodurans is exposed to a high dose of ionizing radiation, a number of genes are induced that lack readily identifiable homologues among known prokaryotic proteins (Liu, et al., supra, 2003; Tanaka, et al., Genetics 168:210-233, 2003). Among these is the gene designated DR0423 (Q9RX92). This locus is one of the most highly induced genes in Deinococcus following γ-irradiation, expression increasing 20-30 fold relative to an untreated control. Although originally annotated as a “hypothetical” protein (White, et al, Science 286(5444):1571-1577, 1999), a more detailed analysis (Iyer, et al., BMC Genomics 3(1):8, 2002) has identified an evolutionary relationship between DR0423p and the important eukaryotic recombination protein Rad52 (P06778). Rad52 is part of a larger family of proteins exhibiting structural similarity but little sequence homology, including the prokaryotic Redβ.(P03698), RecT (NC 000913.1), and Erf (P04892) proteins (Passy, et al., Proc. Natl. Acad. Sci. USA 96(8):4279-4284, 1999; Iyer, et al., supra, 2002).